Electroless metal deposition or plating is useful for the deposition of a metal or mixture of metals on a non-conductive or dielectric surface in the absence of an electric source. Plating on non-conductive or dielectric substrates covers a wide range of applications, including decorative plating and electronic device fabrication. One of the main applications is the manufacture of printed circuit boards. The electroless deposition of a metal on a substrate usually requires pretreatment or sensitization of the substrate surface to make the surface catalytic to the deposition process. Various methods have been developed to catalyze the substrate.
U.S. Pat. No. 3,011,920 discloses a method to catalyze a substrate by immersion of the substrate in the colloidal catalyst solution prepared by palladium ions with stannous ions to form a palladium-tin colloid. This method requires a step of acceleration after catalyzing the substrate surface whereby the catalyst core is exposed. U.S. Pat. No. 3,904,792 discloses an improvement of colloidal palladium-tin catalysts to provide catalyst in less acidic environments. Hydrochloric acid is partially replaced by a soluble salt of the acid. Such a palladium-tin catalyst systems present a number of limitations. The outer shell of the catalyst colloid (SnCl4)2− is easily oxidized, thus the catalyst particles grow in size and lose their catalytic surface area dramatically.
U.S. Pat. No. 4,725,314 discloses a process for preparing a catalytic adsorbate in aqueous solution using an organic suspending agent to protect the colloid with a maximum dimension not exceeding 500 angstroms. Polyvinyl pyrrolidone may serve as an organic suspending agent.
Because of the high cost of palladium, considerable effort has been focused on the development of non-noble metal catalyst systems. U.S. Pat. No. 3,993,799 discloses the use of a non-noble metal hydrous oxide colloid for treating non-conductive substrates followed by reduction of the hydrous oxide coating on the substrate to achieve at least a degree of activation for subsequent electroless plating. U.S. Pat. No. 6,645,557 discloses a method to form a conductive metal layer by contacting the non-conductive surface with an aqueous solution containing a stannous salt to form a sensitized surface followed by contacting the sensitized surface with an aqueous solution containing a silver salt having a pH in the range from about 5 to about 10 to form a catalyzed surface.
JP10229280A discloses a catalyst solution which is composed of silver nitrate or copper sulfate, as well as an anionic surfactant, such as polyoxyethylene lauryl ether sodium sulfate and a reducing agent such as sodium borohydride. JP11241170A discloses a non-palladium catalyst which includes at least one of iron, nickel, cobalt and silver salt, in conjunction with an anionic surfactant and a reducing agent.
JP200144242A discloses a manufacturing method for preparation of a high dispersing colloidal metal solution with high conductivity, which contains at least one amino group and one carboxyl group. U.S. Pat. No. 7,166,152 discloses a sliver colloid based pretreatment solution comprising three components: (i) sliver colloidal particles; (ii) one or more ions selected from metal ions having an electric potential which can reduce a sliver ion to silver metal in the solution; and (iii) one or more ions selected from a hydroxycarboxylate ion, a condensed phosphate ion and an amine carboxylate ion. Normally, aqueous solutions of colloidal tin-free silver are much more stable than those systems involving stannous ions, which are easily oxidized to tin (IV) with air agitation. Colloidal silver catalyst systems would reduce cost and be less unstable than palladium systems. Such colloidal silver catalyst systems also show promising catalytic properties in electroless plating processes without sacrificing the interconnect reliability. Therefore, a colloidal catalyst system which has a balance of bath stability, adsorption capability and catalytic activity at the same time is desired.